The invention relates generally to systems using ceramic membranes in the production of free O2 and/or N2 gases. In particular, the present invention relates to processes for making O2 and/or N2 without subjecting ceramic membranes to undue stress or to reducing environments. In addition, the invention also relates to processes which use both conventional polymeric membranes or pressure swing adsorption (PSA) systems in combination with ceramic membranes (e.g., hybrid processes). The processes described herein allow for ease in purification and productivity as well as power recovery.
Polymeric, PSA and ceramic membrane filters have all been used for separating selected gases from other gas mixtures. Polymeric membranes are described, for example, in U.S. Pat. No. 5,102,600. PSA systems are disclosed, for example, in U.S. Pat. No. 4,769,047 which discloses the use of pressure swing adsorption to remove ethylene from the purge and recycle it back to the reactor.
Membranes made from certain ceramic materials (for example Yttria doped Zirconia or Gadolinum doped Ceria) are also useful in separating gas mixtures. For example, ceramic membranes that are ionic conductors of oxygen become electrically conductive at elevated temperatures due to the mobility of oxygen ions within the crystal lattice. Since these materials are only conductive to oxygen ions in the presence of an electrical current, an external electric circuit is needed. Further, it is necessary to control the electrical current supply in order to regulate the production of oxygen required. Such an oxygen generation device comprises a membrane of such material to one side of which is supplied ambient air. Oxygen diffuses through the membrane by ionic transport and is recoverable from the other side of the membrane. Oxygen production rate is dependent on the electrical current supply to the membrane.
Although gas separation processes which make use of ceramic membranes may be more efficient than those using polymeric membranes, the ceramic membranes are typically subjected to high voltage, high temperature and high pressure conditions. U.S. Pat. No. 5,547,494 describes the use of electrically driven oxide membranes (e.g., ceramic lanthanum strontium cobaltite membranes in the production of free O2). The voltage required to separate the gases increases as the concentration of the gas to be isolated decreases. U.S. Pat. No. 5,944,814 describes production of N2 using an ion transport membrane under similar operating conditions. Under these harsh conditions, particularly the high voltage required, the ceramic membrane may breakdown, for example by reduction. Reduction potentials for bismuth oxide over a temperature range can be calculated from data available in the literature, for example data contained in Chatterji and Smith (1973) J. Electrochem. Soc. 120:889-893 and Turkdogan (1980) xe2x80x9cPhysical Chemistry of High Temperature Technologyxe2x80x9d, Academic Press.
Thus, currently available systems have limited utility and there remains a need for improved and cost-effective systems and processes comprising ceramic membranes that are capable of operating at acceptable levels of separation productivity.
The present invention includes methods of separating a feed gas stream into elemental oxygen and nitrogen. At least two process stages are employed in which the feed gas stream is passed through at least one ceramic membrane and, optionally, at least one polymeric membrane. Unlike current methods, the present invention requires the same or lower voltage for each passage through the ceramic membrane(s), thereby greatly improving the durability of the ceramic membranes.
Thus, in one aspect, the invention includes a process for separating a feed gas stream containing elemental oxygen and nitrogen to produce purified oxygen and nitrogen gas streams. The process comprises: (a) introducing the feed gas into a first process stage comprising at least one first ceramic membrane; (b) selecting a first flux through the first process stage and providing a first voltage across the first ceramic membrane to drive an oxygen-depleted-nitrogen-enriched gas through a purity control and an oxygen-enriched-nitrogen-depleted gas into a first collection chamber; (c) introducing the oxygen-depleted-nitrogen-enriched gas from step (b) into a second process stage comprising at least one second ceramic membrane and providing a second voltage across the second ceramic membrane to drive the oxygen-depleted-nitrogen-enriched gas through a purity control and oxygen-enriched-nitrogen-depleted gas to the first collection chamber, wherein the second voltage is equal to or less than the first voltage. In certain embodiments, the second voltage is less than the first voltage, for example the second voltage is at least 10% less than the first voltage, or at least about 25%-50% less. In certain embodiments, the second process stage is one of at least two process stages successive to the first stage and each stage utilizes a voltage that is the same as or less than the voltage utilized at the preceding stage. In any of the processes described herein, the feed gas stream can be, for example, air.
In another aspect, any of the processes described herein further comprise the step of (d) driving the feed gas stream through at least one polymeric membrane or at least one PSA system prior to step (a). In certain embodiments, the pressure applied to the feed gas stream is higher in step (d) than in steps (a)-(c). In addition, any of the processes described herein can further include driving the feed gas stream through at least one pressure regulator, at least one heat exchanger and/or at least one filter before or after driving it through the ceramic membrane.
In another aspect, the invention includes a process for separating a feed gas stream containing elemental oxygen and nitrogen to produce purified oxygen and nitrogen gas streams, said process comprising: (a) driving the feed gas stream across at least one polymeric membrane or at least one PSA system and delivering a first oxygen-enriched-nitrogen-depleted gas to a first collection chamber and an oxygen-depleted-nitrogen-enriched enriched gas to a first process stage of step (b); (b) selecting a first flux to drive the oxygen-depleted-nitrogen-enriched gas of step (a) through the first process stage comprising at least one ceramic membrane and providing a first voltage across the first ceramic membrane to drive the oxygen-depleted-nitrogen-enriched gas stream through a purity control and a second oxygen-enriched-nitrogen-depleted gas stream to a second collection chamber; (c) introducing the oxygen-depleted-nitrogen-enriched gas from step (b) into a second process stage comprising at least one second ceramic membrane and providing a second voltage across the second ceramic membrane to (i) drive the oxygen-depleted-nitrogen-enriched gas stream through a purity control; (ii) drive the second oxygen-enriched-nitrogen-depleted gas stream to the second collection chamber; and (iii) drive the oxygen-depleted-nitrogen-enriched gas into a third collection chamber or outlet, wherein the second voltage is equal to or less than the first voltage and wherein the nitrogen enriched gas in the third collection chamber is at least about 99% percent pure. In certain embodiments, the second voltage is less than the first voltage, for example at least 10% smaller or at least 25%-50% smaller. In certain embodiments, the second process stage is one of at least two process stages successive to the first stage and each stage utilizes a voltage that is the same as or less than the voltage utilized at the preceding stage. In any of the processes described herein, the feed gas stream can be, for example, air. Additionally, any of the processes described herein can further comprise driving the feed gas stream through at least one additional polymeric membrane or PSA, at least one filter and/or at least one heat exchanger before or after driving it through the at least one ceramic membrane. In yet other embodiments, the pressure on the feed gas stream is reduced prior to driving it through the at least one ceramic membrane, for example using one or more pressure regulators. In certain embodiments, the pressure is below about 7 bar or below about 5 bar.
These and other embodiments of the present invention will readily occur to those of ordinary skill in the art in view of the disclosure herein.